Our studies on adhesion among chick embryo neural cells have indicated that initial binding between nerve plasma membranes involves the function of a cell surface polypeptide with a molecular weight of about 150,000. Monovalent Fab fragments from specific antibodies against this molecule inhibit aggregation of neural cells in suspension and fasciculation of neurites in vitro, and perturb the histotypic sorting of retinal cells in aggregates. During the next year our studies will focus on the chemical characterization of the neural cell adhesion molecule, comparison of its structure to an adhesion molecule from enbryonic liver, and a detailed morphological analysis of the function of the neural cell adhesion molecule in developing tissue. These studies should clarify further the molecular mechanisms of initial cell-to-cell binding, and help define the role of cell adhesion in formation of complex embryonic tissues.